> strat199–(at)–ol.com (Strat1995) writes: >You can drain the voltage from the caps by connecting a resistor from one of >the hot lugs on the cap to ground. I use a 100k.

When power tubes are in place and heated up, they will usually drain most of the B+ quickly when the amp is powered down. This relates to output tubes only…preamp stages will not discharge the ps caps efficiently. If you can get the voltage down to reasonable levels (< 80v or so), you'll still get a nice jolt, but it won't kill you. This is still no guarantee.

Unheated output tubes won’t discharge a supply: if you flip an amp’s power on and off quickly, the supply will stay hot. If you leave the standby in ‘off’ position, the output tube plates probably won’t be connected to the first cap in the supply, so count on that cap being hot. NOTE: cap ‘inrush’ current thru the standby switch is not good, so the cap is usually before the switch.

Some amps use two lower voltage caps in series to get the higher voltage rating for the first supply cap. In this case, there should be resistors (Fender uses 220k’s) for balancing the voltage across the two caps. Those resistors will help to discharge the caps, but the time constant will be relatively long.

Cap voltage decays exponentially thru a resistor, so the first ‘half’ of the B+ will be discharged rel. quickly. This is good.

If I know I will be working on an amp a lot, or if I’m pulling output tubes to tweak output stages, I often solder a resistor in permanently. This can be 100k to 200k or so, but figure out the req’d wattage. B+ squared / ohms = watts. And BE CONSERVATIVE about the wattage. Measure B+ with the standby in the ‘off’ position: If you pull the output tubes or don’t flip the standby, the B+ voltage will increase quite a bit, so overrate that resistor. 500v across a 250K resistor is 1 watt, so use a 2 watt resistor, etc. Note that req’d wattage increases as the SQUARE of the voltage.

Even with the resistor soldered in, you’d best not touch the B+ unless you have to. That’s just a good habit to get into.

Everyone who works on amps gets hit real good eventually. Best make sure that there’s someone nearby who knows exactly what to do if you need help. This is one of the ground rules in many electronics labs! Instruct them to run directly for the breaker box and not to touch you or the amp. If the breaker box is not nearby, install a master switch for your shop in a place where it won’t be blocked.